Ok, so based on your measurements the height of the stand at various points are 100% the same, which is good to hear. I don't understand why there would be the gap that you initially observed, and I'm assuming you filled the tank with water and the gap started to close. The question then becomes how is this happening - ie. what is 'bending' - filling the gap. Was hoping Waterbox would be able to provide some context on what's happening here: what this gap is, and how is the levelling mat fixing it.
Hoping someone else here can chime in here with an engineering perspective.